Why Do Puppies Get Worms?

You bring home a healthy-looking eight-week-old pup, and at the first vet visit the stool sample shows roundworm eggs. It feels like a failure of care. Yet veterinary research consistently finds that nearly all puppies acquire these parasites from their dam, not from their environment. Understanding how and why this happens makes prevention much more straightforward. This article breaks down the transmission routes, the types of worms puppies face, and the standard deworming protocols that stop the cycle.

The Near-Universal Inheritance of Roundworms

Roundworms (Toxocara canis) are the intestinal parasite most frequently diagnosed in puppies. The American Veterinary Medical Association (AVMA) notes that nearly all puppies are born with them because larvae migrate from the mother through the placenta during pregnancy. This is transplacental transmission.

After birth, the mother’s milk carries additional larvae. A nursing puppy ingests these larvae with every feeding. This double route — before birth and during nursing — means that even in clean, well-managed homes, a newborn puppy is extremely likely to harbor roundworms.

Hookworms can follow a similar path. Larvae pass through the mother’s milk, and in young puppies the resulting infection can become severe enough to cause life-threatening anemia. Catching these infections early is why veterinary guidelines emphasize prompt deworming.

Why Mother-to-Puppy Transmission Is So Widespread

It is easy to assume that a visibly healthy mother dog cannot pass worms to her pups. But many adult dogs carry dormant roundworm larvae in their tissues, showing no signs of illness. Pregnancy hormones reactivate these larvae, which then migrate to the developing puppies or into the mammary glands.

This explains why puppies from the most conscientious breeders still often test positive. The mother may have been dewormed during pregnancy, yet a small number of encysted larvae can survive standard treatments. The transmission is not a reflection of poor care; it is a biological reality of the parasite’s life cycle.

• Transplacental route: Larvae cross the placental barrier during the final third of pregnancy, infecting puppies before they are born.
• Transmammary route: Active larvae enter the milk ducts and are swallowed by nursing pups, continuing the infection after birth.
• Environmental ingestion: Puppies that dig in soil or lick contaminated surfaces can pick up hookworm or whipworm larvae from the feces of infected animals.
• Flea-borne tapeworms: Tapeworms are not transmitted from mother to puppy; they require ingestion of an infected flea, making them more common in older dogs that explore outdoors.
• Prey animals: Dogs that eat rodents, birds, or insects can acquire various worms, though this is less common in young puppies still confined to the home.

The distinction matters because roundworm and hookworm control must start almost immediately after birth, while tapeworm and whipworm prevention relies more on environmental management later in life.

How Other Worms Get Into a Puppy’s System

Not all intestinal parasites arrive via the mother. Tapeworms and whipworms are typically acquired from the environment rather than passed in utero or through milk. A puppy that swallows an infected flea can develop a tapeworm infection, and whipworm eggs can survive in soil for years.

Purdue University’s veterinary resources detail how maternally acquired parasite routes differ from environmentally acquired ones. Roundworm and hookworm larvae are actively mobile; they can also penetrate a puppy’s skin directly, especially on the paws or belly when a pup lies on contaminated ground.

Another common path is the fecal-oral route. Puppies explore the world with their mouths. If they sniff, lick, or eat feces from an infected animal — whether from another dog, a cat, or wildlife — they ingest the eggs or larvae. This is why prompt cleanup of yard waste and avoiding dog parks until deworming is complete are standard recommendations.

Note that whipworm infection is listed twice by mistake in the doc — it appears only once in the fact list. Corrected to one row. This table shows why the standard deworming protocol begins so early and repeats frequently.

When to Start Deworming Your Puppy

The AVMA recommends that deworming for roundworms and hookworms begin at 2 weeks of age. Because puppies are constantly exposed through nursing, a single treatment is not enough. Repeat doses are given every 2 weeks until the puppy is weaned, typically around 8 weeks of age. After weaning, the veterinarian will continue the schedule based on the puppy’s specific risk factors.

1. Two weeks old: First deworming dose for roundworms and hookworms. The medication is usually a liquid given by mouth.
2. Four weeks old: Second dose. At this point, a fecal examination may be performed to confirm the presence of parasites.
3. Six weeks old: Third dose. Many breeders handle these early treatments before the puppy goes home.
4. Eight weeks old and beyond: After weaning, the veterinarian will prescribe monthly preventives that cover heartworm and intestinal parasites.

Most puppies should also have a fecal examination at least 2 to 4 times during their first year. Even with perfect deworming, a small number of larvae can remain encysted in tissues, so routine monitoring is important.

Preventing Reinfection and Protecting Your Family

Once a puppy has been dewormed, the environment must be managed to prevent reinfection. Roundworm eggs can survive in soil for years, and hookworm larvae thrive in warm, moist areas. Prompt removal of feces, especially from areas where children play, reduces the parasite load. The AVMA emphasizes that good hygiene — including keeping dogs out of areas where other animals defecate — is essential.

Some intestinal parasites, specifically roundworms and hookworms, are zoonotic: they can be transmitted to humans. The risk is low, but children are more vulnerable because they are more likely to put contaminated hands or objects in their mouths. PetMD explains that tapeworm transmission from fleas requires a flea intermediate host, so flea control is part of prevention against that species.

Adult dogs that hunt or roam should be on year-round preventives. Flea control is particularly important for tapeworm prevention, since the tapeworm cannot complete its life cycle without a flea. Regular fecal checks, even in adult dogs, catch silent infections that could otherwise reseed the yard.

The Bottom Line

Puppies get worms primarily because they inherit them from their mother, either before birth or through her milk. Roundworms and hookworms follow this path nearly universally, which is why early, repeated deworming is standard care. Environmental exposure adds to the picture once the puppy starts exploring. With a solid deworming schedule, routine fecal exams, and basic hygiene, most puppies clear their infections and grow into healthy adult dogs.

If your puppy shows signs like a potbelly, dull coat, diarrhea, or vomiting, a veterinarian should examine a fresh stool sample to identify the parasite and prescribe the appropriate medication. Your vet can also recommend a year-round preventive that matches your puppy’s breed, age, and lifestyle — not all products cover the same worms, so a tailored choice matters more than a generic one.